JG HK416 low FPS issues

[Conker]

I currently have a JG HK416 (2008) on which I installed a Guarder metal hopup and Prometheus 400mm barrel. I'm having amazingly low FPS (can't go thru one side of a Coke can).

Just to confirm what I understood from the barrel-cylinder matching thread, putting a full cylinder (instead of the stock ported one) with this 400mm tighbore should already help a little with my FPS issues? (precision is wonderful, and range is suprising for that FPS so the hopup seems correctly installed).

(Note that the 400mm tightbore will soon be changed with a 429mm one as it doesn't reach the tip of the outer)

Unfortunately, the first time I used it the chrono sucked balls, and I changed the barrel right after so I can't accurately compare before and after.

I don't think that this can explain why a usually 380fps+ gun shoots at ~180 (last chrono reading I got), so I guess that there's also an airseal problem in that.

Other suggestions or things I should check for a better diagnostic?

[Styrak]

No, with a 400mm barrel you should still have a ported cylinder.

Since you installed a new hopup chamber it leads me to believe you either installed it incorrectly, or the rubber incorrectly, or some other reason causing a bad airseal in the hopup/nozzle interface.

[Ayashifx55]

Your piston head is messed up. I had a G&G Veteran M14 that should shoot 410fps and it was shooting 100fps Piston Head died and piston's teeth was all grinded (finished). Therefore, compression sucked. Check air nuzzle as well if its correctly installed.